The present invention relates to portable, electric motor driven tools, more particularly to an improved arrangement for controlling the speed of such tools.
It is common with variable speed, electric motor driven tools to provide some form of speed sensing device which senses the rotational speed of the armature and which, through a suitable control circuit, controls the speed of the motor to the preset value. One difficulty in conjunction with providing a satisfactory speed control circuit is the noise which is inherent in the brush and commutator of the motor and the fact that the tachometer must be relatively small in size so as to fit in the power tool. Such a tachometer must be low in cost and at the same time provide a high signal to noise ratio.
Many forms of tachometer generators used for this purpose comprise a rotor wheel that is provided with alternately polarized areas along its face and which rotates relative to a stationary magnetic core and sensing coil. Because the poles are disposed closely adjacent in each, the rotor with this type of device generally demagnetizes itself and thus the entire generator becomes inoperative. Also, in conjunction with most sensors of this type, the presence of the magnetized stator of the motor causes stray fluxes which generate noise signals and adversely affect the performance of the speed sensing device.
It is, therefore, one object of this invention to provide an improved, simplified and compact tachometer generator for controlling the speed of the electric motor of a power tool.
It is another object of this invention to provide an improved tachometer generator for such an arrangement in which the tachometer generator has a long life and provides a good signal to noise ratio. In conjunction with this, it is also an object to provide a tachometer generator that gives an extremely large output signal for its size.
Briefly, the invention disclosed herein is usable in conjunction with a power tool that has a housing, a speed control circuit for adjusting the speed of the tool motor, and a tachometer generator for supplying an electric signal to the speed control circuit indicative of the actual rotational speed of the tool. The tachometer generator includes a rotor member made of a magneticly permeable material having a central portion in a plurality of teeth extending from the central portion. A sensor member made of a magnetizable material and having first and second end portions and an intermediate portion is disposed with its first end portion at a first location adjacent to teeth of the rotor and its second end portion being disposed adjacent the rotor at a second location which is spaced from the first location. A magnet having opposite poles is also provided. Means fix one of the members for rotation with the output of the motor and the other member against rotation relative to housing for relative rotation of the members upon operation of the tool. In accordance with the invention, the magnet is fixably mounted at one of the two locations so as to be interposed directly between the rotor and the one of the end portions of the stationary member with one of its poles being directed toward the rotor and the other of its poles being directed toward the one end portion whereby a series magnetic circuit is formed which comprises the rotor, a stationary member and the magnet. A coil is disposed about the intermediate portion so as to be responsive to changes in the intensity of the flux passing through the magnetic circuit as the rotor rotates past the two locations.
Other objects and advantages of the present invention will become apparent from the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: